1. Field of the Invention
The present invention relates to a semiconductor device. In addition, the invention relates to a display device having the semiconductor device. In particular, the invention relates to a liquid crystal display device having the semiconductor device and an electronic device having the liquid crystal display device.
2. Description of the Related Art
In recent years, with the increase of large display devices such as liquid crystal televisions, display devices such as liquid crystal display devices and light-emitting devices have been actively developed. In particular, a technique for forming a pixel circuit and a driver circuit including a shift register or the like (hereinafter referred to as an internal circuit) over the same substrate by using transistors made of an amorphous semiconductor over an insulator has been actively developed, because the technique greatly contributes to low power consumption and low cost. The internal circuit formed over the insulator is connected to a controller IC or the like arranged outside the insulator (hereinafter referred to as an external circuit) through an FPC or the like, and its operation is controlled.
In addition, a shift register which is formed by using transistors made of an amorphous semiconductor has been devised as the internal circuit formed over the insulator (see Reference 1: Japanese Published Patent Application No. 2004-78172).
However, there has been a problem in that characteristics of transistors formed of an amorphous semiconductor deteriorate in accordance with an on time or a voltage applied. In order to solve this problem, suppression of characteristic deterioration of the transistors has been devised by connecting two transistors in parallel and sequentially turning on the transistors. (see Reference 2: SID '05 DIGEST PP. 348 to PP. 351).